Electronic switches or switches are used in many electronic applications (for example, in analog and/or digital electronic circuits) for controlling (e.g. enabling or disabling) the transfer of electric signals across them according to specific functionalities to be carried out. The switch comprises, in its simplest implementation, a transistor (for example, of the MOSFET type) in “pass transistor” configuration. In such a configuration, the transfer of electric signals between a first conduction terminal (for example, the drain terminal) and a second conduction terminal (for example, the source terminal) is enabled/disabled by the turning on/off of the transistor. This is typically achieved by driving a control terminal of the transistor (e.g., the gate terminal) with a proper control signal (usually, a signal configured for taking a low level—for example, equal to a ground voltage—or a high level—for example, equal to a supply voltage). Such configuration is not always satisfactory since electric signals with values close to the supply, or ground, voltage are not completely transferred where the switch comprises an N-channel MOSFET transistor (nMOS), or a P-channel MOSFET transistor (pMOS), respectively.
In another implementation, the switch comprises an nMOS transistor and a pMOS transistor in “transmission gate” configuration. In such a configuration, the drain/source terminals of the nMOS transistor are connected to the source/drain terminals of the pMOS transistor (parallel connection), and the gate terminals are driven by complementary levels of the control signal. In this way, the closing of the switch occurs by turning-on both transistors, which enables the complete transfer of the electric signals having values close to the supply/ground voltage across the pMOS/nMOS transistor. The opening of the switch occurs by turning-off both transistors, which disables the transfer of any electrical signal across them.
Although widely used, such an approach may have some drawbacks that preclude wider use thereof, for example, in applications with stringent requirements in terms of electric power consumption, heat dissipation, area occupation, and/or performance. An example of such applications is in analog and/or digital microelectronic circuits (e.g. comprising operational amplifiers), which are integrated in very small areas and are affected by particularly low maximum voltages and currents (e.g. supply voltage of the order of 1-1.3 V).
In such circuits, the use of switches implemented by transistors in “transmission gate” configuration, other than having a high area occupation, also has issues in terms of operation. In fact, the reduction of the supply voltage (and hence the high level of the control signal) results, for the same technology (and hence for the same threshold voltage of the transistors), in decreasing the overdrive voltage (difference between the voltage drop between the gate and source terminals and the threshold voltage) such as not to be sufficient to ensure the correct turning-on of the transistors in any operating condition. For example, in case of a half-swing electric signal, i.e. having an intermediate value between the supply voltage and the ground voltage, the overdrive voltage of the pMOS transistor might be zero (or lower than zero), thereby preventing the turning-on thereof. The overdrive voltage of the nMOS transistor might go to zero before the complete transfer of the electric signal.
An approach to such issues may comprise implementing each switch with a single (typically nMOS) transistor in “pass transistor” configuration, being driven by a control signal whose high level has a value greater than the supply voltage (or overvoltage). Hence, it is able to withstand high voltage drops between the gate terminal and the drain terminal—thus referred to as a high-voltage transistor hereinafter (for distinguishing it from the previous ones, or low-voltage transistors).
Such an approach, although allowing the proper transfer of full-swing electric signals, may not be satisfactory in terms of area occupation, as the high-voltage transistors are larger than the low-voltage transistors, and performance, as the high-voltage transistors, having large size, introduce considerable parasitic capacitances, which limit the switching frequency of the switches. Furthermore, such an approach may require the use of circuit elements (e.g. charge pumps) able to generate the overvoltages, which increases area occupation and electric power consumption.